I've fitted a few of these but not had to do one for maybe 5 or 6 years so this info may be out of date. However, from memory the Saniflo has to be set lower than the shower waste so that gravity clears the shower. The Saniflo has a float which activates the macerator and pump which quickly clears the waste.
Inside the shower the waste water would just run away as normal with the Saniflow kicking in and out as it fills and empties.
These things are (or were) noisy! I know one person who has one and they can't use that particular loo at night because their neighbours can hear it!
We also had one that was problematic. Saniflo sent a rep round pretty quickly, he fiddled with it (what a crap job
) but a week or so later it was playing up again so they replaced it and installed it themselves for no charge. I was quite impressed with their service. It was just a faulty unit.